Twins - class-X

The production of a new individual organism or offspring from the parents is the result of a process called reproduction which can be sexual or asexual. In the case of sexual reproduction, the offsprings tend to carry the genetic information from both the parents which are contained in genes carried by chromosomes. Sexual reproduction produces offspring that resemble their parents but are not identical to them but asexual reproduction does not involve mixing of genetic information and hence tend to give rise to genetically identical  progeny. In the case of human beings, this take place either due to natural cloning where there is the production of identical gametes.

Fraternal twins are developed by fertilization of two eggs (ova) by two sperm to produce two genetically unique children. When a fertilized egg undergo splitting and develops two children having the same genetic information, identical twins are born. Identical twins with joined bodies are called as Siamese twins. 

Fraternal twins occur when two fertilized eggs are implanted in the uterine wall at the same time that is when the mother releases two eggs and both become fertilized by two different sperms.
Identical twins occur when a single egg is fertilized by a single sperm to form one zygote, but the zygote divides into two separate embryos.
Siamese twins are conjoined twins that are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely perhaps because they split very late in the development.

Identical twins are also called as monozygotic twins. They are derived from a single zygote that splits at an early stage during development. They have same genetic make up and are of the same sex. Thus option A is the correct answer.

Identical twins are born when one ovum crosses with one sperm to form one zygote (monozygotic). This zygote later divides into two embryos. These embryos share the same womb. Non-identical twins are born when two ova crosses with two sperms at the same time to form two zygotes (dizygotic).

One fertilized egg (ovum) splits and develops two babies with exactly the same genetic information. They are known as identical twins. In this case, one fertilized egg divides into blastomeres and they become separate. The self-contained halves then develop into two babies, with exactly the same genetic information. 

When one fertilized egg splits and develops into two embryos with the same genetic information is called as identical twins. It is also called as monozygotic as the single zygote is involved in the formation of two embryos. Whereas dizygotic twins are called as fraternal twins in which two zygotes are involved in the formation of two embryos. Thus, the correct answer is option B.

Monozygotic twins are those twins in which is zygote is fertilized by one sperm and during the cleavage, it gets divided into two separate zygotes. These type of twins are exactly identical.

Fraternal twins are produced when two ovums are fertilized by two sperm cells simultaneously.

Fraternal twins are also called as 'dizygotic'. This means there is fertilization of two seperate eggs(ova) by two individual sperms simultaneously. They are implanted in the uterine wall at the same time. They usually develop in two separate amniotic sacs. They are also called as 'biovular' since two eggs form two zygotes.
So, the correct is answer is 'Two ova are fertilized simultaneously'.

Dizygotic twins may occur if two or more oocytes are released in one cycle. If each is fertilized, dizygotic twins may result. Fertility drugs are a common cause for dizygotic twins, but other factors, like family history, maternal age or weight, can also lead to dizygotic twinning. Thus, the correct answer is option D.

• There are two types of twins, Fraternal or dizygotic twins, and monozygotic twins.
  
• When the zygote, during the cleavage, separates and forms two different cells and both of them grow individually as embryos, they form the monozygotic twins.
  
• Monozygotic twins are of the same sex.
  
• When 2 zygotes are fertilized by two different sperms they form the dizygotic twins.
  
• They can be of different sex.